Hello Everyone.
It's really nice to see such a nice forum and helping people.
I'm planning to do Oral/Dental Public Health PhD from Australia. I need your help to decide the topic which is highly demanding in Australia for PhD research.
Your help and suggestions will be highly appreciated in this regard.
Thanking you in advance.

We can't really help with topic choices. You need to develop this by talking to other people in your area (supervisor?) and reading the background literature.

Yes, exactly what ToL said. I think you may not be ready for a PhD yet, as it requires a proposal that highlights a 'gap' in current research, and how you will contribute. So, first step is to read up...a lot, so you know the context you will be working in. That will take some time. In the meantime, look up some potential supervisors in the institution that you want to study in, and see how your proposal can match their research interests. Good luck!

What interests you? What's something that 'turns you on' (as I have had many profs etc ask me in the context of research).

While it might be prudent to pick a topic that is in demand, you might find yourself bored to tears with it. You want to think about something that concerns you, or interests you, and then go from there.